This application is related to and claims priority to U.S. provisional application Ser. No. 60/457,257 filed Mar. 25, 2003, U.S. provisional application Ser. No. 60/474,075 filed May 29, 2003, U.S. provisional application Ser. No. 60/513,809 filed Oct. 23, 2003, U.S. provisional application Ser. No. 60/540,691 filed Jan. 30, 2004, and U.S. provisional application Ser. No. 60/549,211 filed Mar. 2, 2004, the disclosure of each of which is incorporated by reference herein.
This invention was made with government support under AG21842-02 awarded by the National Institute on Aging, and under DK54687-06 awarded by the National Institute of Diabetes and Digestive and Kidney Diseases. The government has certain rights in the invention.